The field of implantable medical devices, including cardiac pacemakers, cardioverters, defibrillators, drug-administering devices, neural stimulators, and the like, has seen considerable advancement over the last several decades. This progress in medical device technology stems not only from advances in medical knowledge, but also to a great extent from recent improvements in the areas of electronics and semiconductors. By taking advantage of the latest technological improvements, manufacturers have been able to increase the overall effectiveness and usefulness of medical devices by increasing their functional capability, sophistication, and complexity.
Early implantable pacemakers, which delivered cardiac stimulating pulses at a fixed rate without inhibition, may seem primitive in comparison to today's state-of-the-art multifunctional pacemakers. Today, pacemakers are available which are programmable into one of various operational modes, from simple single-chamber asynchronous pacing to dual-chamber, synchronous, demand pacing. Many modern pacemakers are capable of automatically adjusting their pacing rates in response to a patient's intrinsic electrical cardiac activity and/or the level of a patient's metabolic demand for oxygen. Most state-of-the-art pacemakers are programmable or multi-programmable, such as with an external programmer which communicates with the implanted device via radio-frequency telemetry. A pacemaker may be programmable with respect to numerous parameters, including pacing mode (DDD, VDD, AOO, etc. . . . ), pacing rate, stimulating pulse width, refractory period, sense amplifier sensitivity, rate-responsiveness to measured physiological parameters, and so on.
Since implantable medical devices are typically implanted subcutaneously and may be implanted in a patient for many years, it has always been an objective in the design of implantable devices that they be as small and lightweight as possible. Often, there is a trade-off between size and functionality of a device. Increasing the functionality of an implanted device can involve increasing the size, weight, and power consumption of the circuitry and/or other functional components required to realize increased functional capability. Power consumption is an important consideration because increased power consumption will be associated with either an increase in battery size and weight, or a decrease in the device's operational life expectancy.
Pacemaker designers and manufacturers have had considerable success in balancing such considerations as pacemaker functionality, sophistication and complexity with considerations of size, weight, and power consumption. In many cases, however, the balancing of these factors comes at considerable economic expense. That is, pacemakers which are fully-featured and highly sophisticated, and at the same time small, light-weight, and long-lived, have become increasingly expensive to design, manufacture, and sell. State-of-the-art pacemakers in some cases cost the consumer three to four thousand dollars or more.
The problems of high implantable medical device costs are further aggravated by the large expense allocable to research and development for these state-of-the-art devices, as well as by the expense associated with extensive clinical investigation and regulatory approval procedures required in most countries.
A fully-featured and highly sophisticated state-of-the-art pacemaker (e.g., a multiprogrammable, dual-chamber, rate-responsive or DDDR pacemaker) may be appropriate for certain pacemaker candidates, such as those with the most serious or complicated cardiac disorders. However, there are also those pacemaker candidates with cardiac conditions for which only a simple pacemaker therapy (e.g., single chamber demand pacing) is indicated. In some instances, a physician can minimize the costs for a pacemaker patient by implanting a small, limited-function pacemaker when a more extensive pacemaker therapy would be unnecessary. That is, a physician can select from among many commercially available pacemakers covering a wide range of sophistication and functionality, and can select a pacemaker whose functionality most closely meets the needs of the pacemaker candidate.
It is believed by the inventors, therefore, that there remains an ongoing need for limited function, low cost implantable pacemakers to meet the needs of pacemaker candidates whose conditions do not require all of the features of more sophisticated and complex pacemakers.
It is also believed that the present invention is applicable to other types of implantable medical devices besides pacemakers, in cases where a limited-functionality version of a given medical device, rather than a more expensive and complex version, may provide appropriate and effective treatment for some candidates.
According to the present invention, several of the sub-systems in an implantable device are down-scaled and/or simplified in order to reduce associated design, manufacturing, and production costs. These subsystems include the master timing oscillator, the programming system, and internal timing systems.
In prior pacemakers, it has been common to measure certain time intervals, such as pacing periods, A-V delay periods, refractory and blanking periods, and so on, by means of a crystal oscillator. Typically, the crystal oscillator produces a clock signal having relatively high frequency, for example 32,768-Hz. This base frequency is often divided, by means of conventional clock divider circuits, to produce several lower-frequency clock signals. In such crystal-controlled pacemakers, all time intervals are measured in units of clock cycles. For example, time intervals may be measured with a binary counter with the 32,768-Hz crystal oscillator signal applied to its clock input; a one-second interval, then, would be measured by causing the binary counter to count from 1 to 32,768.
Many of the time intervals which are relevant to a pacing algorithm (such as base pacing rate, A-V delay, refractory periods, blanking periods, and so on) have durations on the order of magnitude of tens or hundreds of milliseconds or so. For example, a cardiac cycle is typically on the order of one second (1000-mSec), a typical A-V delay value is 120 to 150-mSec, a typical refractory period may be 300-mSec, and a typical upper rate limit may be 400-mSec or so. If time intervals up to one second were to be measured in terms of numbers of cycles of a 32,768-Hz clock, a 15-bit binary counter would be required. On the other hand, if a slower clock were used, a counter with fewer bits would be required to measure such intervals. If a 32,768-Hz crystal oscillator is used, therefore, there will be a cost, in the form of additional circuitry required either to divide down the 32,768-Hz signal to slower frequencies, or to count 15-bit binary values. This additional circuitry could potentially increase the size, weight, and/or current consumption of the implanted device. In addition, the crystal itself is a relatively delicate component which requires care in handling and attachment in the manufacturing process. The implantable-grade crystal is an expensive component and adds "real estate" or size to the pacemaker hybrid circuit. In addition, the crystal oscillator and binary counter divide chain (15-bits) typically consumes several microamps of current from the pacemaker battery.
It is believed by the inventors, therefore, that elimination of the crystal oscillator in an implanted device would be advantageous in terms of the potential reduction of production costs, as well as potential size and weight reduction and increased battery longevity. In accordance with one aspect of the present invention, an ultra low-frequency (i.e., 10-Hz) oscillator circuit is utilized in place of the typically much higher-frequency crystal oscillators found in prior pacemakers. Circuit size is minimized since the number of clock cycles occurring during any relevant time interval will be smaller for a slower clock than for a much faster one. The need for clock dividing circuitry is also avoided.
Another aspect of implantable medical devices which is a subject of the present invention is the programming and telemetry system. In many prior implantable devices, programming is accomplished by communicating to the implanted device digital information identifying at least the parameter to be programmed and the desired parameter value to be programmed. This information is typically in the form of binary digital data which is radio-frequency modulated and transmitted to an antenna within the implanted device. In the implanted device, the modulated signal is demodulated, and the digital information is decoded. For example, the parameter to be programmed might be identified with an eight or sixteen bit binary word, and the desired parameter value may be represented with a second eight or sixteen bit binary word. Other information, such as an identification of the implanted device, verification codes, error correction codes, access verification codes, and so on, may also be communicated to the device during programming. In addition, the transmitted information may include initialization data to reduce the possibility of inadvertent programming of the device. One programming transaction, therefore, can involve the communication to the implanted device of tens or even hundreds of binary digital bits.
Typical telemetry systems require an antenna and a magnetic reed switch to allow communication with the external programmer. Both of these components are fragile and require special handling in the manufacturing process. Additionally, both components require hybrid "real estate" and thus increase the size and cost of the pacemaker.
A radio frequency telemetry system, while having the advantage of allowing large amounts of data to be communicated to the implanted device very quickly, can be costly not only in economic terms, but also in terms of the space it consumes in the implanted device, the power consumed in receiving and demodulating the radio frequency signals, and its weight. It is believed by the inventors, therefore, that it would be advantageous to provide a programming system which does not utilize radio frequency telemetry but still allows communication of the necessary information to the implanted device.
Yet another element of implantable medical devices to which the present invention relates is the memory subsystem for storing programmed parameter values. Many prior pacemakers are primarily digital rather than analog in their operation. Programmed parameter values are provided to such devices in digital binary format, as described above. The digital information may then be stored in random-access memory (RAM) or the like, and extracted therefrom when needed by the device's digital control circuitry. It is believed by the inventors, however, that recently developed analog storage devices may be advantageously applied in implantable medical devices in place of digital storage devices.
For example, in some prior art implantable devices, programmable parameter values are received by and stored in the pacemaker in digital form. In some cases, the stored parameter value may be used by the implanted device in digital form, such as to preset a binary counter or to compare with a counter values. In other cases, the digital parameter value is first applied to a digital-to-analog converter (DAC), and the analog output from the DAC used as a reference voltage for a voltage-controlled component of the device. If a parameter value must be in analog form to be used by the implanted device, the need for a DAC would be eliminated if the parameter value could be stored in analog, rather than digital form.
Additionally, digital data is typically stored in volatile memory (RAM) and may be lost or contaminated (flawed) during EMI, cautery, or defibrillation procedures. To prevent dangerous operation, typical prior art pacemakers have "power-on reset" (POR) circuitry to reset the device's parameters to a "typical" set of values. This circuitry adds complexity and cost to the implanted device and may cause non-optimal function if the RAM contents have been reset.